Remove deprecated GCM web push setup instructions pending GCM's deletion by Google.

[froodian]

Pull Request/Issue Resolution

Description of Change:

These steps (setting up a GCM/FCM project, introducing manifest.json with gcm_sender_id) were only required for older versions of Chrome which rely-upon GCM. Google's recommended integration path since 2016 is to use VAPID authentication instead (which we support transparently, without any user setup required). So, with GCM going away at the end of May, so too should our recommendation of using this legacy setup methodology go away.

Reason for Change:

Closes #ISSUE_NUMBER_HERE

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PR Checklist

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[dkim32]

lgtm!

[EmilyNecciai]

lgtm!